I use a large variety of both at work...I run IT and software development so I've worked with a lot of machines! I am generally constrained and irritated by the full OS X, but to give Apple credit, their iPhone OS is quite impressive for what it is...I love the interface for a mobile device. On a desktop, I'm still dyed in the wool XP. You want an OS that is probably going to be the most universally compatible OS out there for any random selection of software and hardware, XP is the ticket. Vista and 7 are hogs with needless eye-candy, Ubuntu and most recent linux permutations are nice (albeit ideally suited for the hobbyist who likes to tweak, or basic needs users), and Mac OS X is basically like Ubuntu that you pay out the nose for. Albeit with very stylish looking hardware.
But I'm not anti-Apple...other than its overactive tendency to try and change what I am typing, the iPhone OS is fantastic. I was on Windows Mobile 6.1 before that which is probably not a fair comparison. I've tinkered with Android, but only from the emulator in the SDK.